1. Field of the Invention
The present invention relates to a method of and an apparatus for compressing image data having graduation with respect to every pixel block for reducing the amount of data of image information.
2. Description of the Prior Art
In an image processor such as a process scanner, image data is obtained for every pixel, which is a constant small area, by reading an original image. The image data employed in the field of printing are usually larger in amount than those employed in television, and the amount of the information thereof reaches several to tens of mega bites for each image. Such a large amount of image data would require enourous memory capacity if they were stored in a data base as they were; thus the cost for data transmission would increase greatly.
In order to cope with this problem, data compression is executed on the image data for reducing the total amount of information. Several types of data compression methods are well known in the art, such as that using orthogonal transformation and the like.
However, the data compression methods in the art suffer from a trade-off between a compression rate (or compressibility) and quality of a reproduced image produced by the compressed image data. If the compression rate is increased, the quality of the image is lowered. On the other hand, the compression rate is decreased in order to raise the quality of the image. This is not a big issue for the image data employed in a television because the quality of the images of the television is relatively low compared with that employed in the field of printing.
The data compression methods in the art have another issue concerning repetition of data compression and data restoration. If the data compression is execused by high compression rate, the image quality of a restored image obtained by restoring the compressed data is lowered as compared with the orginal image. Thus, when the data compression and restoration are repeated several times, the image quality is gradually lowered.